Au/KDDI is out as their network doesn't use UMTS (3G. Both NTT DoCoMo and SoftBank Mobile use the same 2100 MHz UMTS as Europe).
SoftBank Mobile is probably more willing to deal given their lower market share.
NTT has the better network and would get the iPhone out to more people being the biggest of the operators, but would demand e-cash (it's a big push by them) and perhaps mobile TV.
SoftBank Mobile, on the other hand, would only ask for e-cash and mobile TV.
NTT is the better choice, but it depends on Apple. If they're as difficult to deal with as the Europeans are saying they well be stuck with SoftBank.
 They really hugely hate being called carriers, by the way, outside of North America.
 You need UMTS 3G for the Japanese market anyway, since they don't have GSM, and I think iPhone revision B will also have GPS.